When to Fertilize Tomato Plants: A Guide for a Bountiful Harvest

Properly fertilizing your tomato plants is the key to a successful harvest. Unfortunately, improper handling can also mean an early end to your season. If you're wondering when to fertilize tomato plants, you're on the right track!

Why Fertilizer is Crucial for Tomatoes

You may have heard that tomatoes are considered heavy feeders, requiring a substantial amount of nutrients for successful growth. While this term holds some truth, it can be easily misunderstood.

Tomatoes don't need excessive fertilization (as over-fertilizing carries the risk of root issues). Instead, being heavy feeders simply means they require more frequent fertilization throughout the growing season. Striking the right balance of nutrients will give you the strongest yields. However, be cautious not to overdo it, as providing more than they need can lead to adverse effects such as root burn and yellowing leaves.

Best Times for Tomato Fertilization

I prefer fertilizing tomato plants twice each season. The first time is shortly after planting (but not at the time of planting), and the second time is before they begin fruiting.

While you can fertilize the soil when transplanting seedlings to their final home, this does pose a risk of fertilizer damage as the roots are still acclimating to the new environment. It's best to let transplanted tomatoes adapt gradually without a sudden surge in nutrients that may exacerbate transplant shock symptoms.

This doesn't mean you shouldn't consider soil health and nutrients at the time of planting. Mixing compost, bone meal, or worm castings with the soil at the bottom of the planting hole and around the base of the plant provides ideal conditions for growth without the risk of over-fertilization.

I typically apply the first round of fertilizer about a month after planting and the second round as they start fruiting to aid in producing larger and better-quality fruits. Late-season fertilization can negatively impact fruit development, so it's best to plan the second fertilization early.

What Fertilizer Do Tomatoes Need?

Understanding the NPK (nitrogen, phosphorus, and potassium) values of fertilizers is crucial for providing what plants need at the right time. Additional secondary and micronutrients like calcium, magnesium, and iron are also essential for plant health and robust fruit development.

At planting, tomatoes need a significant amount of nitrogen for leaf production—an essential component of plant energy production. A balanced 10-10-10 or 10-5-5 fertilizer works well depending on the soil's nitrogen content. As the plant begins fruiting, switch to a fertilizer with higher phosphorus and potassium content, such as 5-10-10, to concentrate the plant's energy on fruit production.

Regularly observing plants and periodic soil tests can identify nutrient deficiencies that need addressing. Unnecessary nutrient applications can cause more harm than solving issues, so understand the problem you're dealing with before making any modifications.

How to Fertilize Tomato Plants

The application method of fertilizer depends on its type. Liquid fertilizer can be directly applied to the soil around the plant. Sprinkle slow-release fertilizer around the soil near the stem and gently incorporate. After application, water the plant slowly and deeply to allow the granules to break down and release nutrients.

If you're using liquid fertilizer, you may need to apply it regularly every few weeks after fruiting, as these nutrients can be washed away through watering. Powdered fertilizers provide nutrients in small doses over time, limiting the need for frequent applications.

Always water tomatoes before fertilizing. Under-watered plants may absorb fertilizers faster than they need, potentially causing damage.

Signs of Over-Fertilization and Under-Fertilization in Tomato Plants

If you're planting and feeding tomatoes for the first time, understanding signs of nutrient imbalance helps prevent growth issues or, worse, having no fruit to harvest at the end of the season.

Common signs of under-fertilization include slow growth and yellowing leaves—especially older, lower leaves, indicating a lack of nitrogen. Purple or red discoloration on the underside of leaves may also indicate phosphorus or potassium deficiency. While these issues are not particularly common unless your soil is severely nutrient-deficient, they are issues to be mindful of.

Over-fertilization can lead to a range of problems. Excessive nitrogen results in lush green leaves but no fruits, as the plant directs energy to leaf production rather than flowering and fruiting. Fertilizer burn (scorched or wilted leaves, along with yellow or brown discoloration) is another common symptom.

In both cases, the first step to resolving the issue is to test the soil. This allows you to understand nutrient levels in the soil and make wiser decisions about the type and amount of fertilizer to apply. By doing so, you can ensure a thriving tomato crop and a fruitful harvest at the end of the season.